1. Field of the Invention
The present invention relates to an image capturing apparatus and a control method thereof.
2. Description of the Related Art
As an apparatus for generating an image, there is a digital camera employing a commonly used image sensor such as a CMOS, a CCD, or the like. However, a commonly used image sensor such as a CMOS, a CCD, or the like has a dynamic range that is as narrow as at most approximately 50 to 70 decibels. Thus, it is difficult for such a digital camera to generate an image that expresses the brightness as seen by the human eye. For example, when an image of a landscape having great contrast is shot with such a digital camera, blocked-up shadows in dark portions and blown-out highlights in bright portions occur.
Japanese Patent Publication No. 7-97841 discloses an exemplary technique for solving this problem. According to the technique disclosed in Japanese Patent Publication No. 7-97841, proper-exposure portions are respectively extracted from an image (low-exposure image) that is overall darker than an image with proper exposure (proper brightness) and an image (high-exposure image) that is overall brighter. Then, the extracted proper-exposure portions are composited, so that an image having a high dynamic range is obtained. This sort of technique is referred to as high dynamic range (HDR) image compositing or the like.
FIG. 10A is a diagram schematically illustrating an HDR image compositing mode. This diagram shows a state in which, with HDR image compositing, blocked-up shadows in a background dark portion and blown-out highlights in a highlight portion (the letter A portion) are suppressed. Furthermore, during moving image shooting, this HDR image compositing is successively repeated as shown in FIG. 10B. When HDR image compositing is performed as shown in FIGS. 10A and 10B, it is possible to finally obtain an image that is closer to what is seen by the human eye than an original image (each image obtained during shooting) is.
Recently, optical zoom lenses are widely used in many image capturing apparatuses including video cameras. During moving image shooting with such an image capturing apparatus while performing HDR image compositing, if a zoom operation using an optical zoom lens is performed, a low-exposure image and a high-exposure image have different angles of view as shown in FIG. 10C. As a result, a problem as shown in FIG. 10D occurs in which a difference appears during a zoom operation between the angles of view of images that are to be composited, and the composite image that is to be output becomes blurred. This problem is apparent particularly when the zoom speed is high.
Thus, some of conventional image capturing apparatuses limit the zoom speed within a range in which image blur is not apparent, during moving image shooting while performing HDR image compositing. Accordingly, with conventional image capturing apparatuses, it is impossible to achieve both image blur suppression and high-speed zoom operation during a zoom operation.